1. Field of the Invention
This invention pertains to a process and a device for the electrodialytic regeneration of an electroless metal deposition solution, particularly electroless nickel-hypophosphite plating solutions.
2. Description of the Related Art
Electroless plating of substrates has been known for many years. Electroless deposition typically occurs over both metal and plastic substrates and is performed for both aesthetic and functional purposes. The most widely used electroless plating solutions are electroless copper and electroless nickel. Electroless plating with metals is based on an auto-catalytic process, in which dissolved metal ions are reduced to the metal by a reducing agent present in the deposition solution and thus deposited into the work piece to be plated. Generally, electroless plating solutions comprise a soluble source of the metal to be plated, a chelator capable of keeping the metal ions in solution and a chemical reducing agent. For instance, electroless copper plating solutions generally comprise copper sulfate or copper chloride (soluble source of copper ions), chelators such as EDTA, formaldehyde as a chemical reducing agent, and alkali metal hydroxide. Electroless nickel plating solutions generally comprise nickel sulfate or nickel chloride (soluble source of nickel ions), chelators such as gluconic, citric, malic or lactic acids, sodium hypophosphite (chemical reducing agent) and a pH adjuster such as ammonium hydroxide to adjust the pH to between 4 and 7. Electroless nickel solutions also deposit phosphorous, from the hypophosphite in solution, into the metal plated layer.
The following chemical equation describes the plating function of an electroless nickel-hypophosphite plating solution:
Ni SO4+6 Na H2 PO2xe2x86x92Ni+2H2+2P+4Na H2PO3+Na2SO4
Thus in the plating, reaction, dissolved nickel ions and hypophosphite ions are consumed, while the concentration of the oxidation products, orthophosphite (H2 PO3), and sodium sulfate increases in the solution. Although raw ingredients such as nickel sulfate and sodium hypophosphite are added back to the solution as plating continues to maintain the plating performance of solution, the build-up of by-products ultimately limits the useful life of the plating solution.
The age of a bath is typically stated in terms of the number of metal turnovers (MTO(s)) achieved by the solution. One MTO is achieved when the mass of metal plated out of the Solution equals the mass of metal in the original plating solution, without replenishment. For example, if a 100 liter plating bath has 6 g/l of nickel in the solution, then 1 MTO is achieved when a total of 600 gr. of nickel are plated out of the solution. Usually, when an electroless nickel hypophosphite plating solution reaches 6-10 MTO""s, the by-products in the solution have reached such high levels that plating rate and quality are negatively affected to the point where the solution is no longer commercially useable. Therefore, once nickel-hypophosphite baths reach 6-10 MTO""s they are no longer used, and the spent bath is discarded and replaced with a new bath, resulting in wasted materials, high costs and considerable environmental burden. As a result, various methods have been proposed for extending the useful life of these types of plating solutions.
U.S. Pat. No. 5,221,328 describes one such method where the by-product, orthophosphite, is precipitated out as an insoluble salt and removed in a batch mode. The precipitation agents, yttrium and lanthanides, however, are expensive and salts such as sodium sulfate continue to build in the plating solution. In another known method, ion exchange resins are used to separate the useful components (i.e. nickel and hypophosphite) from the by-products, in a batch operation, and the bath is then reconstituted.
U.S. Pat. No. 5,419,821 describes an electrodialytic process for the regeneration of electroless nickel-hypophosphite plating solutions. The solution to be regenerated is cooled and then conducted through a compartment in a electrodialytic cell which is separated from both the anode and cathode compartments by anion-exchange membranes. When an electrical field is applied, orthophosphate, hypophosphite and sulfates are transported through the anion exchange membrane into the anode compartment. The solution from the anode compartment is treated with calcium or magnesium salts to precipitate the orthophosphate and transported to the cathode compartment so that hypophosphite ions can be transported back into the plating solution. A disadvantage is that the interfering sodium and sulfate ions are not removed from the plating solution. In addition, the process is carried out in the batch mode.
Known regeneration processes suffer from the drawbacks that they fail to effectively remove or reduce the levels of all species of by-products, they are operated in a batch mode, and/or involve the addition of expensive precipitation agents. As a result, there exists a real need for a process that can reliably remove or reduce the levels of all species of by-products in a continuous operation at the same time that the solution is being used for plating so that plating is not interrupted with downtime for the regeneration and the solution is kept in a steady state condition. It is an object of this invention to disclose a regeneration process that meets the foregoing need.